FIG. 9 shows an example of conventional thermal printhead. In the thermal printhead X shown in the figure, a heating resistor 93, electrodes 94A, 94B and a protective film 95 are laminated on a substrate 91 formed with a glaze 92. The portion of the heating resistor 93 which is sandwiched between the electrodes 94A and 94B serves as a heating portion 93a. The glaze 92 is made of e.g. glass and bulges in the thickness direction of the substrate 91 in cross section. The glaze 92 has a relatively low thermal conductivity and prevents the heat from the heating portion 93a from unduly escaping to the substrate 91.
In the thermal printhead X, the heating portion 93a is located at a position retreated from a recording medium relative to the electrodes 94A and 94B. A structure like this is also disclosed in FIG. 1 of Patent Document 1 identified below. With this structure, however, of the protective film 95, the portion covering the heating portion 93a is pressed against a recording medium with a relatively small pressure, which hinders an increase in the printing speed.
Moreover, as the printing speed increases, the heating cycle of the heating portion 93a shortens. Since the heating portion 93a is directly formed on the glaze 92, the cycle in which the glaze 92 receives heat also shortens. Thus, during the printing operation by the thermal printhead X, the glaze 92 is kept at a relatively high temperature while repeating temperature rise and temperature drop. As a result, excessive thermal stress is applied on the glaze 92, which may cause cracking of the glaze 92.
Patent Document 1: JP-A-2001-246770